High chromium white cast iron, such as disclosed in U.S. Pat. No. 1,245,552, is used extensively in the mining and mineral processing industry for the manufacture of equipment that is subject to severe abrasion and erosion wear, for example slurry pumps and pipelines, mill liners, crushers, transfer chutes and ground-engaging tools. The high chromium white cast iron disclosed in the U.S. patent comprises 25-30 wt % Cr, 1.5-3 wt % C, up to 3 wt % Si, and balance Fe and trace amounts of Mn, S, P, and Cu.
The microstructures of high chromium white cast iron contain extremely hard (around 1500 HV—according to Australian Standard 1817, part 1) chromium carbides (Fe, Cr)7C3 in a ferrous matrix with a hardness of about 700 HV. These carbides provide effective protection against the abrasive or erosive action of silica sand (around 1150 HV) which is the most abundant medium encountered in ores fed to mining and mineral processing plants.
In general terms, high chromium white cast iron offers greater wear resistance than steels which have been hardened by quench-and-temper methods, and also provides moderate corrosion resistance compared to stainless steels. However, white cast iron has a low fracture toughness (<30 MPa.√/m), making it unsuitable for use in high impact situations such as in crushing machinery.
Fracture toughness is a function of (a) the carbide content, and its particle size, shape, and distribution throughout the matrix, and (b) the nature of the ferrous matrix, i.e. whether it comprises austenite, martensite, ferrite, pearlite or a combination of two or more of these phases.
Furthermore, high chromium white cast iron has low thermal shock resistance and cannot cope with very sudden changes of temperature.
Previous attempts by the inventor to produce a tougher white cast iron by adding quantities of other elements such as manganese to high chromium white cast iron were unsuccessful. Specifically, the various alloying elements in white cast iron, namely chromium, carbon, manganese, silicon, nickel and iron, can partition differently during solidification, resulting in a wide range of potential chemical compositions in the ferrous matrix. For example, it is possible to obtain a white cast iron with a ferrous matrix containing more than 1.3 wt % carbon, but this can result in the presence of embrittling proeutectoid carbides in the microstructure. It is also possible to obtain a white cast iron with a ferrous matrix containing less than 0.8 wt % carbon, but this can result in an unstable austenitic ferrous matrix with a low work hardening capacity. Furthermore, it is possible to obtain a white cast iron with a ferrous matrix containing a low chromium content, which can result in poor corrosion resistance.
This disclosure is concerned particularly, although by no means exclusively, with the provision of a high chromium white cast iron which has an improved combination of toughness and hardness. It is desirable that the high chromium white cast iron be suitable for high impact abrasive wear applications, such as used in crushing machinery or slurry pumps.